Diurnal and seasonal variations regulate plant growth. Many of the effects of seasonal variation on plant metabolism, such as the effects of temperature change, can be attributed to altered enzymatic rates. In the case of transgenic plants, the catalytic activity of an enzyme may be further influenced by the genetic source of the transgene rather than the plant in which it is expressed (Oliver et al. (1993) Mol. Gen. Genet. 239:425-434).
Current commercially available transgenic plants are made tolerant of herbicides by expression of a single enzyme with a narrow functional range of effective temperature optima. See e.g., Light et al. (1999) Weed Sci. 47:644-650; Light et al. (2001) Weed Sci. 49:543-548. These limitations on herbicide resistance are well documented and have been accounted for in the formulations and application procedures of many herbicides. For example, glyphosate tolerant plants harboring the EPSP synthase enzyme CP4 are not fully tolerant of glyphosate for the duration of the growing season. If the glyphosate is used at a wrong time, the plants suffer and yields drop.
Resistance phenotypes may also be subject to additional environmental variations and/or to differential regulation of a resistance gene within plat tissues, organs, cellular compartments etc. Accordingly, methods are needed in the art to improve transgenes such that the encoded enzymes are functional across a broader spectrum of environmental conditions (e.g., temperature, soil acidity, etc.) and/or physiological conditions (e.g., pH, concentration of an enzyme substrate or cofactor, etc.). To meet this need, the present invention provides methods of expressing two or more enzymes that perform a same or similar function in a plant, wherein the two or more enzymes have difference kinetic parameters, to achieve optimal enzyme activity across a range of environmental and/or physiological conditions.